In recent years, during the operation of a gas turbine the ambience of a casing interior being covered by the casing becomes high temperature and high pressure. Therefore, when a gas turbine is shut down without taking any actions after the operation, a convective flow is generated inside the casing because the casing interior has a high temperature ambience, and as a result, a temperature deviation occurs inside the casing interior. To be specific, because the high temperature air moves to an upper level area, the upper level area inside the casing interior becomes higher temperature, compared with the lower level area. Due to effects of the temperature deviation inside the casing interior, the upper half of a turbine rotor becomes higher temperature than the lower half. Consequently, because there arises a difference in thermal expansion between the upper half and the lower half, the rotor becomes warped and thermally deformed due to the difference in thermal expansion. In order to prevent the thermal deformation of the rotor, conventionally, turning operation is performed after the operation of a gas turbine is stopped, so as to rotate the gas turbine at a very slow speed as much as 2 to 5 rpm.
In addition, the temperature deviation that occurs inside the casing interior effects not only the rotor but also the casing. Specifically, the temperature of the upper part of the casing becomes higher than the temperature of the lower part of the casing, leading to a difference in thermal expansion between the upper part and the lower part of the casing in a similar manner as the rotor. Therefore, the upper part of the casing is warped, causing a thermal deformation. The thermal deformation of the casing is referred to as a “cat-back deformation.” When the casing is subject to a cat-back deformation in such a manner as described above and the deformation amount thereof exceeds a permissible amount, an incident of contact occurs between the casing and the rotor in turning operation. In order to prevent an incident of contact, the present applicant suggests a gas turbine in which the temperature inside the casing interior is equalized by increasing the revolution speed of the rotor so as to perform spinning operation instead of turning operation when a temperature difference occurs between the upper part and the lower part of the casing. (See Patent Literature 1.)
Patent Literature 1: Patent Application Laid Open as H6-2570.